1. Field of the Invention
The present invention relates to duplexers used in RF stages of mobile phone devices, for example, and elastic wave devices which can be used as the duplexers. More particularly, the present invention relates to a duplexer having improved isolation between a transmission terminal and a balanced terminal of a reception bandpass filer and an elastic wave device which is used for the duplexer.
2. Description of the Related Art
Duplexers have been in widespread use in RF stages of mobile phone devices for the purpose of reducing the number of components thereof. For example, Japanese Unexamined Patent Application Publication No. 2003-347964 discloses a duplexer 1001 having a circuit configuration illustrated in FIG. 16.
In the duplexer 1001, a transmission filter chip 1003 and a reception filter chip 1004 are mounted on a substrate 1002. An antenna terminal 1005 is connected to an external antenna 1006. The transmission filter chip 1003 has a ladder circuit configuration including a plurality of series arm resonators S1 to S4 and a plurality of parallel arm resonators P1 and P2. The series arm resonators S1 to S4 and the parallel arm resonators P1 and P2 are each defined by surface acoustic wave resonators. The transmission filter chip 1003 includes a transmission terminal 1007 at one end thereof. A transmission signal is input from the transmission terminal 1007 and supplied to the antenna 1006 via the antenna terminal 1005.
On the other hand, the reception filter chip 1004 is connected to the antenna terminal 1005 via a matching circuit 1008. The reception filter chip 1004 is defined by a surface acoustic wave filter device and has a balanced-unbalanced conversion function. That is, an unbalanced input terminal 1009 of the reception filter chip 1004 is connected to the antenna terminal 1005. The reception filter chip 1004 includes first and second balanced terminals 1010 and 1011.
In addition, the reception filter chip 1004 is defined by a surface acoustic wave filter device having a plurality of IDTs. The configuration of the surface acoustic wave filter device is herein schematically illustrated. One end of a first IDT 1012 disposed at the center is connected to the unbalanced input terminal 1009, and the other end of the first IDT 1009 is connected to a ground potential. One end of each of the second and third IDTs 1013 and 1014 is electrically connected to a ground potential, and the other end of each of the second and third IDTs 1013 and 1014 is electrically connected to the first and second balanced terminals 1010 and 1011, respectively.
Although not described in Japanese Unexamined Patent Application Publication No. 2003-347964, in a configuration which has been conventionally used, the matching circuit 1008 includes a SAW resonator that is connected between the antenna terminal 1005 and the unbalanced terminal 1009, and a coil that is connected between a node between the surface acoustic wave resonator and the antenna terminal 1005 and a ground potential.
In the manufacturing of the matching circuit 1008, when the above configuration including the SAW resonator and the coil is mounted inside the substrate 1002 in order to decrease the size thereof, in the conventional duplexer, isolation characteristics between the first and second balanced terminals 1010 and 1011 and the transmission terminal 1007 of the transmission filter chip 1003 may be deteriorated. This may decrease the attenuation in a pass band of the transmission filter chip 1003 and in attenuation-frequency characteristics of the reception filter chip 1004.